As is known, numerical switching telephone exchanges or private branch exchanges of the digital type, the so-called PABX's (Private Automatic Branch Exchanges), incorporate integrated interface circuits adapted to drive the voice transmissions over plural telephone subscriber lines.
Such integrated interface circuits, e.g. of the kind referred to in the pertinent art as SLIC's (Subscriber Line Interface Circuits) and manufactured by the Applicant, are arranged to intervene between the telephone subscriber lines and the remaining circuitry internal to the exchange, and operative to supply given line voltage and current to the telephone line according to the resistive load on the line.
Each integrated circuit of the SLIC type comprises a first, high-voltage (up to about 140 Volts) circuit portion, connected directly to the subscriber line, and a second, low-voltage (up to 10 Volts) monitoring circuit portion which is connected between the first portion and the remaining circuitry internal to the exchange.
That second circuit portion is intended for monitoring the operation of the first circuit portion, and also operates in bi-directional communication with the exchange internal circuitry to receive and transmit both analog and digital signals relating to the subscriber's telephone messages or to the state of the line or to a state to be acquired by the interface circuit itself.
More specifically, most of the information transmitted by the SLIC integrated circuit to the exchange relate to the state of the line, and may be:
line "off-hook" on account of a subscriber having raised the receiver from its corresponding telephone set; PA1 Line "on-hook", i.e. with the receiver down onto the associated telephone set. PA1 Since in general, roughly 90% of the telephone lines which lead to one exchange would be in the "on-hook" state, there exists a definite demand for minimizing the power absorbed in such "power-down" state by interface circuits of the SLIC type, since these circuits absorb power in definitely larger amounts than the traditional transformer or relay devices which they have supplanted.
To meet this demand, the prior art has proposed a first solution providing for de-activation of the high-voltage circuit portion of the interface circuit with the telephone line in the "on-hook" state, e.g. as described in German Patent Application No. P3621890.1.
However, this prior solution has a drawback in that the function of the SLIC interface circuit is also de-activated in consequence, thereby it becomes impossible to recognize a state newly acquired by the telephone line.
It should be also noted that for such interface circuits of the SLIC type there are four operating modes or states possible, as specified herein below.
A first operating state, referred to as the tone state, is activated by the monitoring circuit portion where a calling signal is to be supplied on the line.
A second operating state, or communication state, is enabled during a phone call, in which case a cross current will appear on the line which is sensed by the interface circuit to identify the "off-hook" state of the line.
A third operating state, or standby state, characterized by a lower power consumption than the previous communication state, is activated with the line in the "on-hook" state. In this third operating state, the single operation performed by the interface circuit will be the detection of the telephone line state in order to possibly change over to the communication operating state.
Lastly, a so-called "power-down" state which cuts down the power consumption to almost zero, the line state (on/off-hook) sensing circuit being also de-activated.
Accordingly, in order to minimize the amount of power absorbed by the interface circuit, while ensuring detectability of the operational conditions of the telephone line, a further solution has been proposed, as disclosed in commonly-owned Italian Patent Application No. 21719-A/87, of which one of us is a joint inventor.
The last-mentioned Application discloses the structure of a line state monitoring circuit, incorporated to the telephone exchange but structurally independent of the interface circuit. By programming the interface circuit for the "power-down" state, the "on/off-hook" states of the line can be detected by that external monitoring circuit.
This prior solution, while substantially achieving its object in the instance of private branch exchanges, does involve increased complexity for the circuits and bears considerably on the procedure for handling messages pertaining to the line state to be addressed to the telephone exchange, which will receive information of the same or complementary type from two discrete circuits, i.e. the monitoring circuit and the SLIC interface circuit.